Method of decolorizing aromatic hydrocarbons



Patented Nov. 25, 1947 METHOD OF DECOLORIZIN G AROMATIC HYDROCARBONS James L. Amos and Kenneth E. Stober, Midland,

Mich., asslgnors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application October 21, 1943, Serial No. 507,100

1 Claim. 1

This invention concerns a method of decolorizing and purifying aromatic hydrocarbons made by pyrolytic processes.

In the manufacture of styrene by dehydrogenation of ethyl benzene the latter is pyrolyzed in vapor phase, and the reaction products are condensed to obtain a crude liquidcondensate. This condensate is composed chiefly of styrene and unreacted ethyl benzene, but also contains substantial amounts of benzene and toluene as secondary reaction products, together with small amounts of impurities of indeterminate composition which discolor the condensate. The crude condensate is separated into its components by a distillation procedure in which a light fraction is first taken off, consisting principally of benzene and toluene. This light fraction, or fore-fraction, is then fractionally redistilled to separate the benzeneand the toluene. Colored impurities in the crude condensate accompanying the benzenetoluene fraction, and these impurities persist through the subsequent redistillation of this fraction for separation of the benzene from the toluene, so that the distillation products are objectionably discolored and thus rendered unsuitable for many purposes. Usual decoloration methods, such as treatment with activated charcoal, sulphuric acid, etc., are unsuccessful in removing the discoloration or involve operating difficulties which render them poorly suited to commercial practice.

It is an object of the invention to devise a method for decolcrizing the aforesaid benzenetoluene fraction to enable the recovery of waterwhite products by distillation of the same. Another object is to provide procedure for recovering toluene of nitration grade from such benzenetoluene fraction. A further object'is to provide a purification method which can be employed co-incidentally with the fractional distillation oi the said mixed fraction. Other objects will also appear from the following description of the invention.

We have discovered that the color-forming impurities may be removed by treating the dis-,

colored benzene-toluene fraction with maleic anhydride, followed by distillation, whereby the dis-' tillate is recovered free from color.. Evidently,

maleic anhydride combines with the colored impurities to form higher boiling compounds which remain in the residue upon distillation of the treated hydrocarbon material. The amount of maleic anhydride necessary to remove the colorforming impurities will vary with the intensity of the color of the starting material, or, in other words, with the amount of impurities present. In practice, we employ 0.05 pound or more, e. g., from 0.1 to 3 pounds and preferably between 0.1 and 1.5 pounds, of maleic anhyride per 100 pounds of the impure aromatic hydrocarbon to be decolorized. Maleic anhydride is soluble in a number of different aromatic hydrocarbons, including benzene, toluene, xylene, ethylbenzene, etc., hence it may be dissolved in the required amount in the impure mixed fraction and the latter then distilled to obtain a colorless distillate. In a preferred embodiment of the invention, the maleic anhydride is added directly to the column of a still as a solution of the same in benzene or' toluene or other suitable solvent.

The distillation may be carried out batchwise or in continuous manner, using a single still or a series or stills, as desired. For example, maleic anhydride may be added to the benzene-toluene fraction in amount sufficient to decolorize the same, and the treated liquid distilled, first taking off a benzene fraction, and then continuing at a higher temperature to distill the toluene fraction. The maleic anhydride reaction products accumulate in the still and are withdrawn with the residue. For commercial operation it may be preferable to carry out a continuous distillation, in which the benzene is separated as overhead from a first still, from which the toluene is drawn off as liquid residue, and the latter distilled in a second still to obtain purified toluene as theoverhead product.

The accompanying drawing is a diagrammatic sketch showing one arrangement of apparatus which may be employed for the continuous separation in colorless form of benzene and toluene from a discolored mixture thereof.

In the drawing, a distilling column I is provided near its midsection with a valved feed inlet 2 and moved through outlet 6. a portion of this con- 25 3 from the boiler 3 to the midsection oi a second distilling column 8 which is provided at its lower end with a boiler i and with a valved outlet line i I. A vapor line i2 leads from the top of column 8 to a condenser I! which is provided with a valved outlet line H. A valved reflux line I! branches irom line I4 and connects with column 8 near its top. Another liquor pipe i8, which also branches from line I, leads to a maleic anhydride dissolver I'll A valved inlet I1 is connected to line it from a source not shown. A valved liquor pipe I! leads from dissolver I I to column I through a header connecting at difl'erent levels above and below inlet 2.

In the operation 01' the apparatus described in the drawing, the discolored benzene-toluene mixture is continuously fed into distilling column i through inlet 2, and a solution of maleic anhydride in toluene is added to the column through line 19 and header 20. At the start of the process, the toluene employed to dissolve the maleic anhydride is red to the dissolver i8 through inlet IT. The benzene vapors from column I pass to condenser from which the condensate is redensate being returned to column l for reflux. The header 20 permits a selective adjustment of the treating procedure. The maleic anhydride solution introduced at any particular level of the column acts to decolorize the refluxing liquid in the column below the point of introduction. If this point is above teed inlet 2 both the benzene fraction concentrating in the upper part oi. the column and the toluene traction concentrating in the lower part of the column may be treated. It is also possible to add the maleic anhydride solution to the top of the column in amount sufilcient to treat the benzene traction, but in amount insufficient to treat the toluene fraction being removed in the lower part of the column, obtaining in this instance a colorless benzene condensate and a colored i'raction comprising the toluene. In commercial operation of the process the benzene.taken of! from column i is usually returned to the general styrene process to be alkylated to ethyl benzene which in turn is dehydrogenated to form styrene, and such returned benzene ordinarily need not be decolorized. Accordingly, it will be suflicient merely to decolorize the toluene traction, in which case the maleic anhydride solution will be introduced into column I at a level below feed inlet 2.

Thus, when the maleic anhydride solution is introduced in suiilcient amount at a level above inlet 2, the distilled benzene in the overhead stream will be colorless. The higher boiling constituents, including toluene and the maleic anhydride reaction products, accumulate as liquid in the lower part of the column and are continuously removed at the bottom through pipe 8. Ii suiiicient maleic anhydride has been added to Y the column, the liquid withdrawn at'the bottom will be colorless. However, it some color remains, additional maleic anhydride solution may be added to the bottom stream. The latter is fed to column 8, where toluene is distilled from the mixture, while the higher boiling residues, including the maleic anhydride reaction products, are,

withdrawn at the bottom of the column. The

vapors from column 9 pass to condenser from which colorless toluene is removed at outlet l4. One portion of the condensate is withdrawn as product, another is directed back to column} as reflux material, and another portion is passed to the maleic anhydride dissolver II. The cycle now being complete, the necessity for the addition oi. toluene through inlet I1 is obviated.

the feed inlet 2, in which case only the toluene willbe obtained colorless in carrying out the puriflcation process. The purified and distilled toluene so recovered under correct operating conditions, providing suflicient maleic anhydride to remove impurities as well as sumcient fractionation in the distilling columns, responds to the specifications for nitration grade.

The following example illustrates results secured in carrying out the principle of our invention, but is not to be bonstrued as limiting the same.

Example 1 A discolored benzene-toluene mixture obtained as a light fraction in distilling a crude styrene condensate, containing 55.8 per cent by weight of benzene, 38.4 per cent of toluene, and 5.8 per cent oi ethyl benzene, was fed continuously into the midsecticn of a distilling column at the rate of pounds per hour. The liquid was distilled at a head temperature sufflcient to distill benzene, and with reflux at the rate oi! 3'75 pounds per hour, 1. e. with a reflux ratio of 5:1. A 10 per cent solution of maleic anhydride in toluene was added to the column below the feed inlet at the rate of 0.45 pound maleic anhydride per hour. The benzene distillate had a yellow color, but was of better than 99 per cent purity. The still liquor from the bottom of the column contained 85.9 per cent of toluene, by weight, 13.2 per cent of ethyl benzene, 0.5 per cent oi. benzene, and 0.4 per cent 9! non-volatile material. Upon fractional distillation of the liquor colorless toluene oi' nitration grade is obtained.

Benzene, toluene, and other aromatic hydrocarbons made by other pyrolytic processes and containing colored impurities which tend to distill together with the hydrocarbons may be decolorized with maleic anhydride in a manner similar to that described above. For example, benzene and monoand poly-alkyl benzenes obtained from coal tar may be treated with maleic anhydride and distilled batchwise or in continuous manner to produce colorless distillates.

Other modes of applying the principle of our invention may be employed, change being vmade as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent or such stated step or steps be employed.

We therefore particularly point out and distinctly claim as our invention:

In a continuous distillation of a mixture of benzene and toluene containing a colored impur ity incidental to their manufacture and which tends to distill together with the benzene and toluene, the steps which consist in iractionally distilling said mixture in a continuous manner to distill benzene from the mixture and leave toluene in the residue while adding to the distilling column a solution of maleic anhydride and a solvent therefor selected irom the class consisting of benzene and toluene at a point in the column below the feed inlet of the impure mixture where the toluene is refluxing in concentrated form, the maleic anhydride being thus added in amount at least chemically equivalent to the amount of colored impurity in the traction refluxing at this point, withdrawing from the column a liquid traction containing the toluene, and distilling the latter to separate substantially colorless toluene therefrom.

'JAMES L. AMOS.

KENNETH E. STOBER.

REFERENCES CITED The following references are of record in the file of this' patent:

UNITED STATES PATENTS Number Name Date 2,104,956 Stern et a1 Jan. 11, 1938 1,701,988 Torrey et a1 Feb. 12, 1929 OTHER REFERENCES Publication by Bachman and Goebel in Journal of American Chemical Society, vol. 64, pages 787- 790, April 1942, copy of which is found in the 10 Scientific Library of this ofiice and in Div. 31,

Class 260, sub. 681.5.

"Elements of Fractional Dlstillation,'by C. 8..

Robinson, pages 67, 109, 113, 114, published by McGraw-Hill Book Co., New York, 1922, copy of which is found in Div. 25 of this ofice. 

